1. Field of the Invention
The present invention relates to the telecommunication field and more in particular to the art of error correction which could be originated during the transmission of signals. Still more in particular, the present invention relates to an improved FEC decoder and an improved method for decoding signals.
2. Description of the Prior Art
As it is known, the Forward Error Correction (FEC) is a technique by means of which redundancy is transmitted together with transported data, using a pre-determined algorithm. The receiving device has the capability of detecting and correcting multiple bit errors that could occur during transmission thanks to the redundancy. The signal transmitted with FEC is more “robust” thus allowing operators to build up longer distance connections without the deployment of many repeater stations.
In other words, in order to overcome transmission errors and packet loss, many telecommunication systems use forward error correction (FEC). In general, FEC schemes transmit extra data which can be used at the receiving end to re-create any corrupted or lost packets. For instance, FEC has been applied to CD-ROMs to compensate for scratches, and used in satellite and deep-space transmissions, since the broadcast is in only one direction (i. e. the receiver is incapable of asking for retransmission).
Many of these systems use the Reed-Solomon algorithm, which is primarily designed to take an arbitrary stream of data and restore any corrupted section therein, with the appropriate amount of error correction contained in the stream. In order for the algorithm to recover data that has been corrupted or lost in an arbitrary location, the algorithm must include enough error correction to compensate for the fact that some error correction may not be received at the receiving end as well (i. e. the algorithm needs to be able to account for the fact that both data and error correction may be lost).
As it is known, a FEC code based decoder is designed for correcting a fixed maximum number of symbols in a codeword. For instance, a Reed-Solomon FEC could correct up to 4 symbols in a codeword. This means that in case the number of errored symbols is less than or equal to four, all the symbols can be corrected and the codeword can be correctly reconstructed.
The problem arises when the number of errored symbols is higher than the fixed maximum number, for instance, higher than four in the above example. If this is the case, the errors could be wrongly corrected and, furthermore, a high number of additional errors can be introduced.
In general terms, the telecommunication channels could be divided into two different categories as far as the introduction of errors is concerned. Channels introducing error bursts (for instance the radio channels) and channels introducing errors in a random manner (for instance, optical channels). If the second category of channels is considered, the number of errored bits in 4 symbols is around 4 (typically, 4 or 5).
If the symbols are 8-bit long, the average number of introduced errors (in bits) is (symbols×8)/2, where “symbols” is the number of correctable symbols. Thus, in the above example, the mean number of introduced errors is (4×8)/2=16 and the maximum number is 32. The man skilled in the art will easily understand that the introduced errors are well higher than the average number of errors (≅4-5 errors). In the present example, if the number of errors that the decoder tries to correct is well higher than 4 or 5, it is highly possible that a wrong correction is performed and new errors are introduced.